The prototype device developed at the National Institute of Standards and Technology (NIST) is a silicon chip topped with circular mats of carbon nanotubes standing on end. The miniature cryogenic radiometer builds on NIST’s previous work using nanotubes to make an ultra-efficient, highly accurate optical power detector, and advances the ability to measure laser power delivered through fiber for calibration customers.

“This is our play for leadership in laser power measurements,” said project leader John Lehman. “This is arguably the coolest thing we’ve done with carbon nanotubes. They’re not just black, but they also have the temperature properties needed to make components like electrical heaters truly multifunctional.”

The circular patch of carbon nanotubes on a pink silicon backing is one component of NIST’s new cryogenic radiometer, shown with a quarter for scale. Gold coating and metal wiring have yet to be added to the chip. The radiometer will simplify and lower the cost of disseminating measurements of laser power. Courtesy of Tomlin/NIST.
Laser power is typically measured by tracing it to fundamental electrical units. Radiometers absorb energy from light and convert it to heat; then the electrical power needed to cause the same temperature increase is measured. NIST researchers discovered that the mini-radiometer can accurately measure both laser power (brought to it by an optical fiber) and the equivalent electrical power within the limitations of the imperfect experimental setup. The tests were performed at a temperature of 3.9 K using light at the 1550-nm telecom wavelength.

The small circular forests of tall, thin nanotubes, called VANTAs (vertically aligned nanotube arrays) have several desirable properties. Most notably, they uniformly absorb light over a broad range of wavelengths, and their electrical resistance depends on temperature. The nanotubes are versatile, performing three different functions in the radiometer: One VANTA mat serves as both a light absorber and an electrical heater, and a second VANTA mat serves as a thermistor. The mats are grown on micromachined silicon chips, an instrument design that is easy to modify and duplicate. In this application, the individual nanotubes are about 10 nm in diameter and 150 µm long.

Ordinary cryogenic radiometers use many types of materials and are difficult to make. They are typically hand-assembled using a cavity painted with carbon as the light absorber, an electrical wire as a heater and a semiconductor as the thermistor. Furthermore, these instruments must be modeled and characterized extensively to adjust their sensitivity. The equivalent capability in NIST’s mini-radiometer, on the other hand, is easily patterned in the silicon.

NIST plans to apply for a patent on the chip-scale radiometer. Simple changes such as improved temperature stability are expected to improve the device’s performance. Future research may also address extending the laser power range into the far-infrared region, and integration of the radiometer into a potential multipurpose NIST-on-a-chip device.